Abstract : A powerful analytical tool has been developed for in-situ, real-time measurement of the thermally and mechanically induced transient decomposition processes of energetic materials, and has been applied to the new, insensitive high explosive, nitro-1,2,4-triazol-5-one (NTO). Modification of the vacuum chamber of a laser-desorption mass spectrometry apparatus has enabled us to perform simple-shear and compressive fracture of pressed pellets directly beneath the sampling region: fragments spontaneously emitted from fractured NTO pellets are measured by single-photon ionization (SPI) time-of-flight mass spectrometry. The fracture-induced fragments of NTO are dominated by a single peak, m/z 99, which is completely absent in either the thermal- or laser-desorption spectra obtained in the same apparatus. This difference suggests that under the marginal stress of subcritical mechanical fracture, intermediates of mass 100 and/or 101 are diverted to the closed-shell, relatively stable species at mass 99, which then tends to accumulate, rather than going on to produce a next generation of unstable intermediates at m/z 85, 71, and 43. Thus, in its first use, this combination of techniques has enabled identification of intermediates not previously observed in NTO decomposition and allowed us to propose a reasonable sequence of reactions that involves all of these intermediates, reconciles a substantial portion of previous slow thermal decomposition data, and potentially explains the initiation insensitivity of NTO as resulting from diversion to a relatively stable 'dead-end' species.